Ball grid arrays are well known in the art and are available commercially. Such an array comprises a plastic or a ceramic substrate with a pattern of pads or recesses, each providing a receptacle for a solder ball. The substrates, hereinafter referred to as carriers or devices, are available in strips containing a repetitive pattern of pads or recesses, and individual segments of the strip can be detached and used separately.
The task of populating the recesses reliably is a difficult one and a number of procedures to accomplish reliable solder ball placement have been devised. Pat No.5,431,332 issued Jul. 11, 1995 discloses one such procedure. The patent describes a station in a manufacturing line for the placement of solder balls on a ball grid array carrier. The carrier has a pattern of solder pads with an adhesion layer on the solder pads. A stencil, having a thickness of between one fourth and five fourths the diameter of a solder ball, is placed on the carrier. The stencil has a pattern of apertures which matches the pattern of pads on the carrier. The apparatus employs a pallet to move the carrier to the stencil and employs a solder ball dispenser for pouring solder balls in bulk over the stencil. The apparatus also includes a vibration device coupled to the station for urging the solder balls into the apertures in the stencil and onto the adhesive layer above the solder pads. Thereafter, a column of air moves across the surface of the stencil to remove the excess solder balls.
One problem with the aboved describe procedure is that a solder ball is less than 0.030 inch and the stencil is about one half that thickness. The stencil is seprated from the ball grid array a distance of about 0.015 inch to allow for flux. It is very difficult to maintain the stencil in a plane in registry with the ball grid array at such a small separation. Consequently, a solder ball may be misplaced between the stencil and the ball grid array, not in registry with solder pads. The dispensing of the solder balls in bulk and the use of an air knife to remove excess solder balls from the stencil also do not provide a sufficient arrangement for maintaining the tolerances required for the stencil. A mechanical arrangement for maintaining the stencil in a plane is much to be preferred. Such an arrangement would be similar to that of a silk screen process and would entail the movement of a solid reservoir container or solder ball bin over the stencil. But this again has it's own problems. Specifically, a problem exists as to how to maintain the ball bin attached to the stencil avoiding any gap therebetween and, again how to maintain the stencil in a plane.